Most telecommunication systems of today are based on a cellular concept, where communication with a user equipment (UE) within a certain geographical area can be provided with a base station (BS), in some contexts also referred to as a nodeB or an enodeB. In early systems, it was common to divide the radio resources between base stations within interfering distances and each base station could more or less control their own traffic independently of other traffic. However, in recent systems, the overlap of the coverage areas of the different base stations has increased, e.g. by introducing BSs of different “magnitudes”. It has also been more common to allow one and the same UE to simultaneously communicate with more than one BS.
Such configurations have put demands on the BSs to be synchronized in time with each other. If one UE is expected to be able to communicate with more than one BS simultaneously, the different nodes have to operate at a common time scale. One possible approach is to establish a communication between all BSs in order to exchange synchronization information. However, such communication typically steals capacity from the actual load traffic.
One example of such an approach can be found in e.g. the published patent application US 2008/0101514 A1, in which a method and an arrangement for synchronization are disclosed. The disclosure described transferring in a digital data transfer system of timing messages within control data carried in a protocol data unit.
Well synchronized BSs can also be used for positioning purposes. User Equipments may be localized, e.g. for emergency situations, utilizing radio signaling characteristics from different BSs. Requirements in the US demand that 67% of all calls should be possible to localize within 100 m and 95% within 300 m. Some of the positioning methods require synchronization between the different BSs. The traveling time over 100 m for a radio signal is 330 ns, which gives a hint of what degree of synchronization is required.
Synchronization and exact knowledge of time is also used in other areas, such as remote sensors, finance industry and electrical power distribution.
Another possibility to achieve synchronization is to rely on information available through different types of Global Navigation Satellite Systems (GNSS). See e.g. “Understanding GPS: Principles and Applications, E. D. Kaplan (ed.), Artech House, 1996, sect. 2.7-2.7.3, pp. 54-56. Such a system comprises a number of, typically earth stationary, satellites that are positioned at well known positions and that are operating together with a well-known synchronization. The satellites transmit signals that are possible to receive at the earth. By having knowledge of the arriving times of a number of signals from different satellites, triangulation processes can be performed in order to calculate a position of the receiving node, based on information about the positions of the satellites and the transmission times according to the synchronized satellite time. If the receiving node itself is not a-priori synchronized, also this can be computed if signals from a sufficient number of satellites are detectable. A possibility to obtain synchronization between different units and nodes in a communication system is to let each of the units synchronize relative a GNSS time.
So far, three approaches of implementation of GNSS-based synchronization have been discussed. All three solutions can be based on discrete components or assembled modules. The most straight-forward approach is to provide a GNSS receiver built-in within each network element. However, such a solution is relatively inflexible. Since the GNSSs develop fast, new systems as well as updated functions and signaling protocols are expected to come. A built-in GNSS receiver is then difficult or at least expensive to update. Furthermore, a dedicated GNSS antenna port has to be provided. In cases where the synchronization is provided by other means, such GNSS receiver and antenna port will be unutilized.
Another solution is to locate the GNSS receiver at or in close proximity of the GNSS antenna. Synchronization signals are then transmitted to the network element, typically using a dedicated port, which typically cannot be used for other purposes if GNSS synchronization is not used. The GNSS receiver operation is also restricted to signals from one single GNSS antenna. Since the GNSS receiver is positioned at the antenna, mounting as well as updating procedures may also be troublesome.
It is also feasible to provide a stand-alone GNSS receiver somewhere between the GNSS antenna and the network element that needs the synchronization. However, since space often is optimized in and close to network elements, it is often troublesome to obtain a compact site installation. Also here, dedicated ports are used for synchronization signals, which ports typically cannot be used for other purposes if GNSS synchronization is not used.